The present invention relates to an electric compressor used in a vehicle air conditioner.
A typical electric scroll compressor used in a vehicle air conditioner has a stationary scroll and a movable scroll. The stationary scroll is fixed to a housing, and has a base plate and a volute portion. The movable scroll has a base plate and a volute portion. The volute portions inter mesh. When an electric motor accommodated in the housing is driven and the movable scroll orbits, each of compression chambers defined between the volute portions is moved toward the center of the volute portions, while the volume of the compression chamber is progressively decreased. Accordingly, refrigerant gas is compressed.
Japanese Laid-Open Patent Publication No. 2002-295369 discloses an electric scroll compressor that lubricates an orbiting mechanism that permits a movable scroll to orbit relative to a stationary scroll. The scroll compressor of the publication also improves the sealing property of compression chambers against a compression reaction force in a thrust direction applied to the movable scroll. Specifically, the scroll compressor has a back pressure chamber at the back side of the base plate of the movable scroll. The back pressure chamber surrounds the orbiting mechanism. Lubricating oil the pressure of which corresponds to a discharge pressure is retained in a bottom portion of a discharge chamber. The lubricating oil is guided to the back pressure chamber so that the movable scroll is urged toward the stationary scroll. Accordingly, the sealing property of the compression chambers is improved.
In the electric scroll compressor of the publication, lubricating oil that lubricates the orbiting mechanism and increases the back pressure falls by the self weight down to a motor accommodating chamber through an oil bleed passage having a constriction. The lubricating oil is then temporarily retained in a reservoir formed in the bottom of the motor accommodating chamber. Thereafter, the lubricating oil is sent to a suction side of the compression mechanism, which includes the volute portions of the stationary scroll and the movable scroll, through a conveying passage.
When used in a vehicle air conditioner, the above described electric scroll compressor has the following drawbacks. The reservoir for lubricating oil is formed in the bottom of the motor accommodating chamber. Therefore, when a significant amount of liquid refrigerant returns to the compressor from a refrigeration circuit, mixture of the lubricating oil and the liquid refrigerant stays in the lubricating oil reservoir. The coils of the motor and other components can be impregnated with the mixture. In a typical electric compressor, polyol ester (POE) is used as lubricating oil, so that the lubricating oil exerts a sufficient insulating performance even if mixed with liquid refrigerant. An electric compressor using such lubricant oil has no drawbacks when applied to an ordinary air conditioner.
However, in vehicle air conditioners, polyalkylene glycol (PAG) is predominantly used as lubricating oil for belt driven compressors. When mixed with liquid refrigerant, PAG significantly degrades the insulating property of the mixture liquid. When performing maintenance of such a vehicle air conditioner, PAG can be mixed with liquid refrigerant. If wire connections and stator coils are impregnated with such mixture of the lowered insulating property, leakage of electricity can occur.
Such leakage of electricity can occur not only in electric scroll compressors, but also in electric swash plate type compressors and electric vane compressors.